Human chronic cholestatic liver diseases (i.e., cholangiopathies) trigger the proliferation and damage of cholangiocytes that are the epithelial cells that line the biliary apparatus. These cholangiopathies cause morbidity and mortality and are a major reason for liver transplantation. During the progression of cholangiopathies, proliferation of cholangiocytes is critical for the maintenance of biliary mass and function. Proliferating cholangiocytes serve as a neuroendocrine compartment during liver disease pathogenesis and secrete and respond to hormones and neuropeptides, which contribute to the autocrine and paracrine pathways that modulate liver inflammation and biliary fibrosis associated with cholestatic liver diseases. The elucidation of the factors regulating the proliferative responses of cholangiocytes to cholestasis is critical for the development of therapeutic strategies for the treatment of cholestatic liver diseases. The overall objective of the current application is to determine the role that the local cholangiocyte renin-angiotensin-system (RAS) plays in the maintenance of biliary mass during cholestasis. We have recently obtained novel data indicating that cholangiocytes express a local RAS and that chronic administration of angiotensin II (Ang II) increases the proliferation of normal rat cholangiocytes and enhances the proliferation of cholangiocytes during extrahepatic cholestasis induced by bile duct ligation (BDL). We have also obtained unique in vitro preliminary data demonstrating that mechanical stress, which occurs during extrahepatic obstructive disorders, induces cholangiocyte proliferation via mechanical activation of the angiotensin type 1 receptor (AT1). Based on preliminary findings, we propose the novel central hypothesis that the RAS plays a key role in regulating biliary mass. As such, cholangiocyte proliferation during cholestasis is stimulated via ligand (Ang II)- and mechanical stress-dependent stimulation of the AT1 resulting in the activation of ERK1/2 MAPK pathway-dependent signaling mechanisms triggering activation of cholangiocyte proliferation and the local RAS via coordinated signaling of stress activated MAPKs JNK1/2 and p381. Our proposed work will focus on three specific aims designed to test the following working hypotheses: (i) activation of AT1 by the ligand Ang II plays a key role in the modulation of cholangiocyte proliferation;(ii) mechanical stress-induced activation of AT1 and the local RAS plays an unique role in the modulation of cholangiocytes proliferation;and (iii) ligand and mechanical activation of AT1 and the RAS plays a key regulatory role for controlling the proliferation of cholangiocytes during in vivo models of liver disease. Knowledge of the intracellular mechanisms through which the RAS modulates cholangiocyte proliferative responses to cholestasis will play an important role in the development of therapeutic strategies for the treatment of liver diseases (e.g., cirrhosis, PBC, PSC, and other ductopenic diseases). PUBLIC HEALTH RELEVANCE: The health relatedness of this application is that effective treatments are lacking for chronic cholestatic liver diseases, such as primary biliary cirrhosis (PBC), and primary sclerosing cholangitis (PSC). Chronic cholestatic liver diseases cause damage to the bile ducts of the liver. The rationale for our research is that the successful completion of the studies can ultimately be expected to provide a greater understanding of cholestatic liver disease progression and increase opportunities for the development of novel treatment paradigms for chronic liver diseases.